Wireless Power Charging

ABSTRACT

A current sensing circuit and a minimum operating frequency for a wireless power transmission system is presented. A method of measuring current through a wireless power transmit coil, includes receiving a signal from a switching circuit into a sampling circuit; filtering the sampled signal from the sampling circuit; biasing the filtered sampled signal, wherein the biasing occurs only when the sampling circuit is active; and amplifying the biased signal to provide a transmit coil current signal. A method of measuring current through a wireless power transmit coil, includes receiving a signal from a switching circuit into a sampling circuit; filtering the sampled signal from the sampling circuit; biasing the filtered sampled signal, wherein the biasing occurs only when the sampling circuit is active; and amplifying the biased signal to provide a transmit coil current signal.

TECHNICAL FIELD

Embodiments of the present invention are related to wirelesstransmission of power and, in particular, to wireless power charging.

DISCUSSION OF RELATED ART

Mobile devices, for example smart phones, tablets, wearables and otherdevices are increasingly using wireless power charging systems. Ingeneral, wireless power transfer involves a transmitter driving atransmit coil and a receiver with a receiver coil placed proximate tothe transmit coil. The receiver coil receives the wireless powergenerated by the transmit coil and uses that received power to drive aload, for example to provide power to a battery charger.

Typically, a wireless power system includes a transmitter coil that isdriven to produce a time-varying magnetic field and a receiver coil,which can be part of a receiving device such as a cell phone, PDA,computer, or other device, that is positioned relative to thetransmitter coil to receive the power transmitted in the time-varyingmagnetic field.

However, wireless power transmission provides for multiple challenges.One is monitoring the TX coil current, which can represent up to 82% ofthe transmitter power losses.

Therefore, there is a need to develop improved ways to operate wirelesspower transfer transmitters to provide for more efficient transfer ofpower.

SUMMARY

In accordance with embodiments of this disclosure, a current sensingcircuit is presented. Further, a system for setting a minimum operatingfrequency for a wireless power transmission system is presented.

In some embodiments, a wireless power transmitter includes an inverterdriven that includes series coupled MOSFET transistors driven by gatecontrol signals, the inverter driving configured to drive a transmitcoil coupled to a switching node between the series coupled MOSFETtransistors; and a current sensing circuit coupled to receive a signalfrom the switching node. The current sensing circuit includes a samplingcircuit coupled to receive the signal from the switching node when thesampling circuit is turned on, an amplifier coupled to receive a sampledsignal from the sampling circuits, a filter coupled to the samplingcircuits to filter the sampled signal; and a bias circuit coupled tobias the sampled signal at the amplifier, wherein the bias circuit isturned on when the sampling circuit is turned on.

A method of measuring current through a wireless power transmit coil,includes receiving a signal from a switching circuit into a samplingcircuit; filtering the sampled signal from the sampling circuit; biasingthe filtered sampled signal, wherein the biasing occurs only when thesampling circuit is active; and amplifying the biased signal to providea transmit coil current signal.

A method of operating a wireless power transmitter includes operating awireless power system that includes the wireless power transmitterproviding power to a wireless power receiver at a first frequency;determining a zero-voltage switching (ZVS) deadtime at first frequency;lowering the first frequency until the ZVS deadtime is above a maximumZVS deadtime; and setting a minimum operating frequency from the firstfrequency.

A wireless power transmitter includes an inverter that includes an uppertransistor in series with a lower transistor and coupled between aninput voltage and a ground, the upper transistor being driven accordingto a UG signal and the lower transistor being driven according to a LGsignal; a TX coil coupled to a switching node where the upper transistorand the lower transistor are connected; a capacitor coupled between theswitching node and ground; an analog front end coupled to receive theinput voltage, ground, and a switching voltage at the switching node,the analog front end providing signals that indicate the level of theswitching node; and a processor with a ZVS control process, theprocessor executing instructions to operate the wireless powertransmitter to provide power to a wireless power receiver at a firstfrequency; determine a zero-voltage switching (ZVS) deadtime at thefirst frequency; lower the first frequency until the ZVS deadtime isabove a maximum ZVS deadtime; and set a minimum operating frequency fromthe first frequency that is above the maximum ZVS deadtime.

These and other embodiments are discussed below with respect to thefollowing figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a wireless power transmission system.

FIG. 2 illustrates a wireless power transmitter on which embodiments ofthe present disclosure can be implemented.

FIG. 3 illustrates a conventional current sensing circuit.

FIG. 4 illustrates a current sensing circuit according to someembodiments according to the present disclosure.

FIG. 5 illustrates another wireless power system with further aspects ofthe wireless power transmitter according to this disclosure.

FIG. 6 illustrate resonant tank gain curves as a function of frequencyfor a system as illustrated in FIG. 5.

FIG. 7 illustrates functionality of a conventional wireless powersystem.

FIG. 8 illustrates functionality of a wireless power transmitteraccording to some embodiments of the present disclosure.

FIG. 9A illustrates details of the wireless power transmitterillustrated in FIG. 5 illustrating aspects according to the presentdisclosure.

FIG. 9B illustrates a process operating on the wireless transmitterillustrated in FIGS. 5A and 5B according to some embodiments of thepresent disclosure.

FIG. 10 further illustrates aspects of the wireless power transmitterillustrated in FIGS. 5 and 9A according to the present disclosure.

FIG. 11 illustrates further details of operation of a wireless powertransmitter illustrated in FIGS. 5 and 9A.

These figures are further discussed below.

DETAILED DESCRIPTION

In the following description, specific details are set forth describingsome embodiments of the present invention. It will be apparent, however,to one skilled in the art that some embodiments may be practiced withoutsome or all of these specific details. The specific embodimentsdisclosed herein are meant to be illustrative but not limiting. Oneskilled in the art may realize other elements that, although notspecifically described here, are within the scope and the spirit of thisdisclosure.

This description illustrates inventive aspects and embodiments shouldnot be taken as limiting—the claims define the protected invention.Various changes may be made without departing from the spirit and scopeof this description and the claims. In some instances, well-knownstructures and techniques have not been shown or described in detail inorder not to obscure the invention.

FIG. 1 illustrates an example wireless power transmission system 100 onwhich embodiments according to the present disclosure can be executed.As illustrated in FIG. 1, a wireless transmitter 102 is coupled to acoil 106 and a wireless receiver 104 is coupled to a coil 108. Coil 106is driven by wireless transmitter 102 to produce a time varying magneticfield that in turn induces a current in coil 108. Coil 108 is coupled towireless receiver 104, which can receive the power transmitted throughthe time varying magnetic field from wireless device 102.

Wireless receiver 104 can be included in any device with wireless powerfunctions. Many phones, laptops, tablets, and other devices include awireless power function. In many cases, these devices can both receiveand transmit wireless power. In some examples, wireless transmitter 102may be a stationary wireless power charger.

Some embodiments of the present invention allow for measurement of thecoil current in a wireless power transmitter. The coil current in awireless power transmitter should be monitored. In some cases, thecurrent through the coil can represent up to 82% of the losses in thetransmitter. Monitoring the current can help predict the powerdissipation associated with the current, which is used to improvedetection of foreign objects. Detection of foreign objects is a criticalaspect of the responsibility of the wireless power transmitter accordingto the Qi wireless power protocol.

Other embodiments of the present invention allow for using dead timemeasurements in zero-voltage switching to set a minimum allowablefrequency to avoid entering an inversion area. Using a dead-timemeasurement can allow operation with minimum allowable frequencies muchclose to a peak frequency for the particular system involving the powertransmitter and power receiver system.

FIG. 2 illustrates an example of a wireless power transmitter (PTx) 200on which embodiments of the present invention may be executed. PTx 200may be formed on a single IC that is used to drive a transmit coil 222.PTx 200 receives power from an outside source into a power block 208.Power block 208 includes DC/DC converters and other power handlingdevices such as, for example, Buck converters, power filtering,low-dropout regulators (LDOs) and other devices to provide for the powerneeds of PTx 200. As is illustrated in FIG. 2A, PTx 200 includes aninverter/driver 210 coupled to drive an external inverter 216 thatdrives transmit coil 222 according to instructions from a processor 204.

Processor 204 can be any processing device that is capable of executinginstructions to perform the functions described in this disclosure. Insome embodiments, processor 204 is coupled through interface 202 toanother device that provides instructions, in which case the functionsdescribed in this disclosure are performed by a combination of processor202 and a device coupled through interface 202 with processor 204.Processor 204 can include any combination of microcomputers,microprocessors, state machines, or other circuitry that perform a partor all of the functions described in this disclosure.

Processor 204 can be coupled to a memory 206. Memory 206 can be anycombination of volatile and non-volatile memory that stores data andinstructions executed in processor 204. Memory 206 also includes anyregisters that are used in operation of processor 210.

FIG. 2 illustrates a wireless power transmitter 250 that illustratescurrent sensing aspects of embodiments according to the presentinvention. As is illustrated in FIG. 2, wireless power transmitterincludes PTx 200, which in some embodiments can be formed on a singleintegrated circuit (IC), and external circuit as further describedbelow.

As is illustrated in FIG. 2, PTx 200 includes a processor 204 that iscoupled to power block 208, inverter/driver 210, and a currentmeasurement control 212. PTx 200 is coupled to an inverter 216 whereinverter/driver 210 drives transistors in invertor 216 that drivescurrent through TX coil 222. Inverter 216 may include a full-bridge(formed with four transistors) or a half-bride inverter (formed with twoseries-coupled transistors) to drive current through a TX coil 222.Power to the inverter can be provided by power block 208, which can becontrolled by processor 204 to control the output power of thetime-varying magnetic field generated in coil 222. As is discussedfurther below, processor 204 can also control the wireless power outputby controlling different characteristics of the MOSFET PWM waveform,such as duty cycle, phase or frequency at5 which inverter 216 is driven,or other characteristics.

As is further illustrated in FIG. 2, the switching node or switchingnodes 220, that are the nodes between inverter 216 and coil 222, aremonitored by a current sense circuit 218. Current sense circuit 218 isturned on or off by current signal from control block 212 in PTx 200. Asis illustrated, processor 204 can turn current sensing circuit 218 on oroff with a control signal from current control block 212. Further, asignal indicative of the coil current, I_TX coil, can be received fromcurrent sensing circuit 218 into an analog-to-digital converter (ADC)214 to be digitally read into processor 204 for further processing. Themeasured current is used in various algorithms executed on processor 204to control the power output of the wireless power transmitter formed byPTx 200 in communication with TX coil 222. The measured current isfurther used to monitor power loss in the wireless transmitter. In someembodiments, instead of being received into ADC 214, an analogprocessing and comparator circuit may be used to monitor thetransmission current levels.

In some embodiments, PTx 200 may include current sensing 218incorporated on the same IC. However, as illustrated in this disclosure,current sensing 218 is an external circuit to PTx 200.

Monitoring the current in a low-cost manner is challenging. Currentsensing circuit 218 may use a MOSFET drain-source on resistance (RDSON)of a low-side transistor of inverter 216 and a single op-amp to form adesirable low-cost method, especially in multi-coil systems that use acommon point for sensing the coil current. Such a design can help saveon component cost and design complexity. However, existing technologieshave challenges balancing response time with noise filtering when usinglow-cost amplifier circuitry. Noise associated with the turn-on andturn-off of the sampling MOSFETs is a particularly challenging problemto address.

FIG. 3 illustrates a conventional system 300 for measuring the TXcurrent as discussed above. FIG. 3 illustrates an inverter 302, whichcan be that illustrated as inverter 216 in FIG. 2. Inverter 302 includesseries coupled MOSFET transistors 306 and 308. The gates of transistors306 and 308 are driven, for example by a driver such as driver 210 asillustrated in FIG. 2, to drive current through a TX coil that iscoupled to the switching node 320. As illustrated in FIG. 3, transistor306 is driven by a GH signal from driver 210 and transistor 308 isdriven by a GL signal from driver 210. Switching node 320 is theembodiment switching node 220 as illustrated in FIG. 2, which in thisexample is the node between transistors 306 and 308. Inverter 302 mayalso include filter circuit 310 to filter the power supplied to a TXcoil coupled to switching node 320. Circuit 312 is a zero-voltageswitching (ZVS) circuit that slows down the slew rate of SW node 320 tohelp prevent electromotive interference (EMI). As is illustrated, acurrent sensing circuit 304 is coupled to SW node 320 to measure thevoltage at SW 320 that results from the RDSON of transistor 308 and thecurrent I_TX through a coil coupled to SW 320.

FIG. 3 further illustrates a current sensing circuit 304 that is coupledto switching node 320. As is illustrated in FIG. 3, current sensingcircuit 304 includes series-coupled sampling transistors 314 and 316coupled between switching node 320 and an amplifier 340. Transistors 314and 316 are sampling MOSFETs with gates controlled by a control signal,for example the control signal as generated by current control 212 ofPTx 200. As is illustrated in FIG. 3, amplifier 340 is powered at node326 by a power output LDO 2 from, for example, power block 208 from PTXIC 200. A filtering circuit 318 may further be coupled to the couplingbetween transistors 314 and 316 and amplifier 340. The output ofamplifier 340 is coupled to the signal I_TX coil signal input to PTx200, as illustrated in FIG. 2, through an output filter 328. Outputfilter 328 can be a peak detect filter.

Although FIGS. 2 and 3 illustrate current sensing circuits 218 and 304,respectively, as external to the IC formed by PTx 200. As is discussedabove, current sensing circuits 218 and 304 may, in some cases, beintegrated into a single IC with PTx 200. Further, bias circuit 330 maybe calibrated for improvement gain by adjusting the voltage LDO1 at noload operation.

FIG. 3 illustrates a current sensing circuit 304 using a synchronousMOSFET sensing circuit. In some embodiments, amplifier 340 can be alow-cost amplifier. The circuit design then has an input offset 330 tokeep the sensed voltage at amplifier 340 within the common mode range ofthe low-cost amplifier 320. As is illustrated in FIG. 3, offset 330 isformed with a biased resistive divider formed by series coupledresistors 322 and 324 where the input to amplifier 340 is coupled to thenode between resistors 322 and 324. The bias comes from voltage LDO1,which again can be received from power block 208 in PTx IC 200. In someembodiments, LDO1 can be a 1.8V bias voltage.

However, the capacitors, specifically capacitors 332 and 334, offiltering circuit 318 in this arrangement must be kept small to allowthe system to stabilize quickly to the sensed voltage level. Thespecific issue is that the bias voltage disturbs the voltage stored onthe filter capacitors 332 and 334 by increasing their offset when thesampling MOSFETs 314 and 316 are off, which can negatively impact theoperation of current sensor 304.

FIG. 4 illustrates a system 400 with a current sensing circuit 404according to embodiments of the present disclosure. In comparison withsystem 300 and current sensing circuit 304 illustrated with FIG. 3, biascircuit 330 is replaced by a bias circuit 430. In bias circuit 430, atransistor 402, with gate coupled to the control signal, is turned onwith transistors 314 and 316. Transistor 402, which is a MOSFET, iscoupled to a voltage divider formed by series-coupled resistors 422 and424. The node between resistors 422 and 424 are input to amplifier 340.Further, filter 318 is replaced with filter 418, which includescapacitors 432 and 434.

As current sensing circuit 404 a synchronous sampling circuit, the biasdriven by voltage LDO1 (e.g., 1.8 V) can be controlled by the same gatesignal that controls the sampling of the input signal from switchingnode 320 by MOSFET transistors 314 and 316. Placing a MOSFET transistor402 in series with the bias resistors 422 and 424 achieves the goal ofnot negatively affecting the capacitors 332 and 334 of filter 318 duringthe times when MOSFET transistors 314 and 316 are off. In someembodiments, bias circuit 430 can be calibrated, for example byadjusting the voltage on LDO1. Such calibration can be accomplished, forexample, with no load.

Further, the filter capacitors 432 and 434 can be much larger thanfilters 332 and 334 of filter 318 of conventional current sensingcircuit 304 since they are no longer biased when sampling is notoccurring. The end result is a cleaner output signal I_TX coil on theboard and improved measurements of the current signal.

In particular, in current sensing circuit 404 current only flows throughthe bias resistors 422 and 424 when current sensing circuit 404 isactive. As discussed above, the current flows when MOSFET transistor 402is turned on by the same gate control signal that turns on the samplingtransistors 314 and 316.

Furthermore, bias on amplifier 340 is only applied when current sensingcircuit 404 is monitoring the voltage at SW node 320, which ismonitoring the voltage at SW node 320. This voltage, for example, can begiven by I_coil*RDSON_308), where RDSON_308 is the drain-source ONresistance of MOSFET transistor 308.

This novel solution elegantly allows for much higher qualitymeasurements by allow for the use of larger filtering capacitors whilesimultaneously reducing the output power by allowing current through thebias circuit only when the current control circuit is active.Consequently, the current sensing circuit 404 allows for a wirelesspower system 400 that monitors coil current with external circuitryusing RDSON (the RDSON of transistor 308). Current sensing circuitincludes an amplifier 340 with inputs that use filters for noisereduction and are biased to align with the specification of amplifier340. Current sensing circuit 404 samples the voltage at switching node320 only when the control signal is activated. The biasing circuitfurther includes a MOSFET transistor 402 in series with the voltagedivider formed by resisters 422 and 432. Transistor 402 is alsoactivated by the control signal, which prevents biasing when currentsensing circuit 404 is inactive.

The current on the coil coupled to SW node 320 can be further improvedwhen correlated other variables, for example the peak voltage on SW node320. This results from an integration of the coil current. Correlatingthe integrated coil current with the peak coil current helps processor200 to calculate the IRMS, which is applied to estimating the AC lossesin wireless power transmitter 250.

One area, among many, where the current through the coil is used formonitoring and controlling wireless power transfer, is with monitoringand controlling switching in the inverter with zero-voltage switching.FIG. 5 illustrates a wireless power system 500 that includes a wirelesspower transmitter 510. Wireless power transmitter 510 transmits wirelesspower through TX coil 222 with PTx 200 driving inverter 216. Asdiscussed above, the transmitted wireless power from transmitter 510 canbe received into a wireless receiver 104 through receive coil 108. As isknown, driver 210 drives transistors in inverter 216 to provide a timevarying magnetic field at TX coil 222. Driver 210 drives inverter 216 ata particular frequency, which may be adjusted to maximize efficientpower transmission to wireless receiver 104. In FIG. 5, current sensingcircuit 218 has been omitted, but may be present to monitor the currentthrough Tx coil 222 by measuring the voltage at switching node 220 asdiscussed above. However, some embodiments may use other current sensingcircuits, including ones incorporated within PTx 200, besides thecurrent sensing circuit 404 as described above.

As is illustrated in FIG. 5, an analog-front-end (AFE) circuit 504 iscoupled to receive the voltage from SW node 220 and provides a signal toa zero-voltage-switching (ZVS) controller 502. ZVS controller 502 isexecuted by processor 204 and monitors the voltage at switching node220. ZVS controller 502 then controls switching in driver 210 so thatswitching of transistors in inverter 216 occurs when the voltage acrossthe switched transistor is zero. This procedure greatly reduceselectromotive interference (EMF) that is caused by switching oftransistors in inverter 216. ZVS controller 502 can also provideinformation regarding the time between when the transistors in inverter216 should be switched to allow for a particular frequency of operationand the time that the transistors are actually switched to provide forZVS operation is the dead time, which is discussed further below.

In accordance with, for example, the QI wireless power protocol,variable frequency control can be used as one method for PTx 200 toregulate the power transfer between the wireless power transmitter andthe wireless power receiver. When operating in a normal operatingregion, lowering the operating frequency increases power and increasingthe operating frequency decreases power delivery.

FIG. 6 illustrates this feature with a graph of resonant tank circuitgain versus frequency for a wireless power system such as system 500 asillustrated in FIG. 5. As is illustrated, tank circuit gain curves 602illustrate the tank circuit resonant gain versus driving frequency forthe wireless power transmission between wireless power transmitter 510and receiver 104 with receiver 104 servicing various loads. Asillustrated in FIG. 6, curves 602 illustrate highest and sharpest curveswith lighter loads and lower, wider curves for higher loads. The highcurve 610 illustrates a 0% load on receiver 104 while the lowest curve612 illustrates a 100% load on receiver 104. The curves between curves610 and 612 illustrates various loads on receiver 104. As illustrated,from top curve 610 to lowest curve 612, the loads illustrated are at 25%load, 40% load, 55% load, 70% load, and 85% load.

FIG. 6 also illustrates the peak gain 608 for each of the curves 602. Asis illustrated, as the load is increased, the peak frequency for eachcurve shifts from lower frequencies to higher frequencies. As is furtherillustrated in FIG. 6, wireless power systems operating in an operatingregion 604 that is higher than the peak gain. As discussed above, whenoperating in normal operating region 604 when the frequency increasesthe power (represented by the gain curve 602) decreases while when thefrequency decreases the power increases.

However, if the system's operating frequency drops below the naturalfrequency (represented by the peaks 608 of the gain curves 602) of thesystem, the control relationship inverts. In inversion region 606, wherethe operating frequency is below the peaks 608 of curves 602, furtherlowering of the operating frequency decreases power delivered towireless power receiver 104. The transition point between operatingregion 604 and inversion region 606, marked by the peak 608 of the curveof curves 602, is difficult to detect. Further, the region of controlinversion also displays hard switching where the electromotiveinterference (EMI) characteristics of the system is increased.Consequently, in most conventional systems the wireless power system isoperated to avoid this situation.

Consequently, conventional systems have some method of setting a lowerbound to the operating frequency which is above the natural frequency ofthe system to prevent control inversion. In FIG. 6, the minimumoperating frequency is represented by the peak frequency 608. In manysystems, the minimum operating frequency is set higher than the peakfrequency 608 of the particular operating curve 602.

FIG. 7 illustrates the conventional setting of a minimum operatingfrequency 702 that is well above the peak frequency 608. FIG. 7illustrates again the gain curves 602 as illustrated in FIG. 6. Asillustrated in FIG. 7, conventional operation sets the minimum operatingfrequency 702 to shift the allowed operating region 604 to frequencieshigher than the minimum allowed operating frequency 702. This, however,leaves a gap 704 between the minimum allowed operating frequency 702 andthe peak frequency 608 in which the system is not allowed to operate.

In the past a given wireless power transmitter design is characterizedon a bench against known wireless power receiver designs operating overa variety of conditions to determine the maximum system resonantfrequency 702. This maximum system resonant frequency 702, whichincludes some safety margin, is loaded into the power transmittersystem, which then monitors the system operating frequency such that thefrequency is never allowed to go below the set minimum operatingfrequency 102 for that wireless receiver. The weakness of this techniqueis that all designs are limited by the worst product the wireless powertransmitter was characterized against. New products could be outsidethis limit and the wireless power transmitter has no way tocompensate—resulting in control inversion, EMI and poor userexperiences. Further, potentially high efficiency areas of operation ingap 704 are inaccessible to the wireless power transmission system.

Operating close to the resonate frequency is desirable to maximizeefficiency and/or delivered power. Achieving this without controlinversion can be very challenging as the minimum frequency can vary withload and with alignment between the wireless power transmitter and thewireless power receiver in the wireless power system.

With reference again to FIG. 5, as discussed above ZVS control 502 inprocessor 204 is operated to monitor dead time and control the operatingfrequency to achieve zero-voltage switching (ZVS). As is illustrated inFIG. 5, PTx 200 includes auto-tuning ZVS dead time control loop 502which controls the switching of transistors in inverter 216 so thatswitching is accomplished when the voltage at node 220 is zero-volts.ZVS control loop 502 may be at least partially operated on processor204, as is illustrated in FIG. 5. As is illustrated in FIG. 5, ZVScontrol loop 502 operating in processor 204 receives signals from ananalog-front-end circuit 504 that receives signals from the switchingnode 220 from inverter 216. ZVS control loop 502 determines thezero-voltage switching dead time from the data received from SW 220, asis further discussed below. Other techniques including monitoring bodydiode conduction time and hard switching characteristic monitoring, arealso possibly included in PTx 200.

As illustrated, PTx 200 lowers the operating frequency while monitoringthe amount of dead time required to achieve a Zero Voltage Switching(ZVS) situation in ZVS control circuit 502. The lowest frequency wherethe ZVS dead time is below a threshold maximum allowable deadtime valuecan be set as the minimum allowable operating frequency. This situationis illustrated, for example, in FIG. 8.

FIG. 8 illustrates a graph 810, which again illustrates the gain curvesillustrated in FIGS. 6 and 7. Consequently, FIG. 8 also illustrates theresonant tank gain versus frequency curves 602 as discussed above. As isillustrated, the peak frequency 608 for each of the resonant curves incurves 602 is illustrated. Also illustrated with graph 810 is graph 812,which illustrates the deadtime 814 as a function of frequency as thefrequency is decreased in the allowed operating region 804. When thedeadtime exceeds a specifically programed maximum time 816, PTx 200stops reducing the operating frequency of the system. Reaching themaximum dead time 816 indicates that PTx 200 has reduced the frequencybelow a point where the system may transition into the Control InversionOperating region 606. Consequently, PTx 200 increases the frequency sothat the dead time 814 is below the maximum dead time 816, where theminimum allowable frequency 806 can be set. This situation allows theminimum allowable frequency 806 to be set according to the actual systemwith real-time characterization and results in a very small unusable gap808. The small gap 808 provides for very little wasted frequency rangein the control of the wireless power system. The maximum deadtime 816can be set around the value above which the wireless power transmittertransitions to control inversion region 606.

FIG. 9A illustrates a process 900 that can be executed by processor 204with ZVS controller 502 according to some embodiments of the presentdisclosure. In some embodiments, process 900 may be operated tocharacterize wireless power transmission system 500 when receiver 104 isbrought in proximity to wireless power transmitter 510. In the exampleprocess 900 illustrated in FIG. 9A first sets the minimum allowablefrequency and then operates the wireless power transmitter 510 whilecontinuing to monitor the deadtime.

As illustrated in FIG. 9A, process 900 starts in step 952 at anoperating frequency that is high enough to be within the operatingregion 804 as illustrated in FIG. 8. In step 956, the deadtime isdetermined. In step 958, processor 204 determines whether the deadtimehas exceeded a maximum dead-time value, which has been preprogrammedinto PTx 200. If not, then processor 204 proceeds to step 954 where theoperating frequency is lowered. Processor 204 then returns to step 956to determine the deadtime with the new operating frequency.

If the maximum deadtime has been reached in step 958, then in step 960processor 204 may increase the frequency slightly to allow for a safetyfactor to set the minimum allowable frequency 806 for the wireless powertransmission system 500. In step 962, PTx 200 then operates wirelesspower transmitter with the minimum allowable frequency 806 as set instep 960, adjusting the operating frequency within the allowed operatingrange as needed. Periodically, PTx 200 may proceed to step 964 where thedeadtime is determined using the current operating frequency. In step966, if it is determined that the deadtime is below the maximumdeadtime, then PTx 200 returns to step 962 for continued operation. Ifthe deadtime is below the maximum deadtime, however, then PTx 200 mayreturn to step 960 to reset the minimum allowable frequency 806 to ahigher value.

Consequently, as illustrated in FIGS. 8 and 9A, PTx 200 lowers itsoperating frequency and monitors the amount of dead time required toachieve Zero Voltage Switching. As illustrated, PTx 200 includesauto-tuning ZVS deadtime control loop 502, although other systems mayinclude other techniques including monitoring body diode conduction timeand hard switching characteristic monitoring, are also possible.

When the frequency is lowered to a point where the deadtime exceeds aspecifically programed maximum time, PTx 200 stops reducing theoperating frequency of the system. Reaching the deadtime limit in step958 indicates that PTx 200 has reduced its frequency too far and is atrisk of entering a region that does not support ZFS (i.e. hard switchingmay occur), indicating operation in the Control Inversion Operatingregion 606 instead of the allowed operating region 804. Consequently, instep 960 the system frequency may be increased to set the minimumallowed operating frequency 806 as illustrated in FIG. 8.

In some embodiments, in steps 962, 964, and 966 PTx 200 continuallymonitors the dead time. If the maximum deadtime is reached, PTx 200 canadjust the minimum operating frequency 806 higher so that system 500does not transition to control inversion region 606.

FIGS. 9B, 10, and 11 illustrate further zero-voltage switching (ZVS) anddetermination of the dead time. FIG. 9B illustrates a further example ofwireless power transmitter 510 as has been illustrated in FIG. 5, and onwhich process 900 may be executed. As illustrated in FIG. 9B, inverter216 is illustrated has a half-bridge converter with series-coupledtransistors 902 and 904 coupled between an input voltage Vin and ground.In this example, inverter 216 is powered by the same voltage that powersPTx 200. Transistor 902 is the “upper” transistor because it is coupledto the input voltage and is driven by an upper gate (UG) signal from PTx200. Similarly, transistor 904 is the “lower” transistor because it iscoupled to ground and is driven by a lower gate (LG) signal from PTx200.

Switching (SW) node 220 is the node between transistors 902 and 904. TXcoil 222 is coupled between switching node and ground. In thisarrangement, a capacitor 914 is coupled in parallel with TX coil 222,between SW node 220 and ground.

Processor 204 can be a microcontroller unit that may operate useregister transfer level (RTL) processing with the ZVS control process502. AFE 504 provide an indication of the SW voltage at SW node 220 withrespect to ground and the input voltage. AFE 504 may, in one exampleembodiment, include comparators 906, 908, 910, and 912, although anynumber of comparators may be used. Comparators 906 and 908 compare thevoltage at SW node 220 with in the input voltage and comparators 910 and912 compare the voltage at SW node 220 with ground. As is illustrated,comparator 906 and 908 provide an indication as to whether or not the SWnode voltage is within a window defined by voltage sources 916 of theinput voltage. Similarly, comparators 910 and 912 provide an indicationto processor 204 as to whether the SW node voltage is within a windowdefined by voltage sources 916.

As discussed above, Zero Voltage Switching (ZVS) occurs when the voltageacross a switch is equal to zero volts when the switch is turned on. Asdiscussed above, the voltage at SW 220 can be given by I*RDSON, where Iis the current through coil 222 and RDSON is the on-resistance oftransistor 914. As is illustrated in FIG. 902, inverter 216 can operatewith transistor 902 on and transistor 904 off, transistor 902 off andtransistor 904 on, or both transistor 902 and transistor 904 off.Inverter 216 will not operate with both transistor 902 and 904 on (whichwould short Vin to ground).

At a transition from transistor 902 being on to transistor 904 being on,after transistor 902 is turned off the LG signal to turn transistor 904on is delayed, creating dead time where no Switch is on. During thedelay time, the coil current through TX coil 222 allows discharge ofcapacitor 914 to naturally change the voltage at SW node 220 from VIN toGround. This process is further discussed in U.S. Ser. No. 10/211,720entitled “Wireless Power Transmitter Having Low Noise and HighEfficiency, and Related Methods,” which is herein incorporated byreference in its entirety. An analogous event occurs at the transitionbetween transistor 904 being on and transistor 902 being on.

ZVS control 502 naturally adjusts the dead time between assertion of theUG signal and assertion of the LG signals. If the current in the coil istoo small to achieve ZVS, dead time is increased. If current is toolarge and the ZVS point is missed the dead time is decreased. FIG. 10illustrates a waveform at SW node 220. A late switching (the deadtimebeing too large) is illustrated in waveform 1002, where the voltagesovershoot. An early switching (the deadtime being too small) isillustrated in waveform 1004, where the waveform does not reach thetarget voltages. Waveform 1006 illustrates optimal switching wherewireless power transmitter 510 naturally reaches the target voltages.

As the operating frequency of wireless transmitter 510 approaches thenatural frequency of wireless power system 500 defined at peaks 608, thecurrent available for slewing SW node 220 approaches zero requiring,longer and longer dead times to prevent a hard switching situation. Thisis further illustrated in FIG. 11. FIG. 11 illustrate an operation 1102where the operating frequency of the system is greater than the peak608, which defines the natural frequency, for system 500. Operation 1104illustrates a situation where the operating frequency is the naturalfrequency of the system. Operation 1106 illustrates where the operatingfrequency is less than the natural frequency of the system.

Operation 1102 illustrates the coil current Icoil 1108 with the UGsignal 1110. Operation 1104 illustrates the coil current Icoil 1112 withthe UG signal 1114. Operation 1106 illustrates the coil current Icoil1116 with the UG signal 1118.

As illustrated in operation 1106, when the operating frequency is lessthan or equal to the system's natural frequency there is no realisticamount of dead time that will achieve ZVS switching. In this region,when UG signal 1118 is switching at time 1120 (i.e. turning from on tooff), the current illustrated in Icoil current 1116 is negative andtherefore is moving in the opposite direction needed for ZVS.Consequently, hard switching is forced to occur, making ZVS impossiblein the assigned switching window. This region is also the controlinversion area 606 as illustrated in FIG. 8.

In operation 1105, the system is operating at the natural frequency. Inthat case, switching at time 1122 occurs when the coil current Icoilillustrated by curve 1112 is zero, there is insufficient coil current toachieve ZVS.

However, in operation 1102, there is a positive coil current atswitching time 1124 there is a positive Icoil current. This results inoperation which can achieve ZVS with a finite delay time.

The Auto-ZVS 502 with AFE 504 detects the lack of ZVS operation andattempts to achieve ZVS by increasing the UG-LG Dead Time. Auto-ZVS willincreased dead time until it saturates its control loop. When the systemdetects that Dead Time has reached its (maximum) value (saturated) thesystem knows the coil operating frequency is below the natural frequencyand it must be increased to achieve ZVS. Well-designed Auto-ZVS systemshave a saturation limit to detect this (or similar) types of events. Inpractical application additional operational information is used toaugment this decision, for example operating mode of the inverters andload current of the system—in order to manage corner operatingconditions.

As discussed above, this detection method allows processor 204 todynamically adjust the operating frequency to be as close as possible tothe natural frequency (to optimize efficiency and/or power delivery) andstill maintain ZVS operation (for low EMI). This method is discussedabove with respect to FIG. 9.

Consequently, in accordance with some embodiments, a wireless powertransmitter changes the operating frequency with a minimum operatingfrequency that is close to the natural frequency. The wireless powertransmitter can change the operation frequency so long as it remainsabove the minimum operating frequency in order to adjust the poweroutput of the wireless power transmitter. The minimum operatingfrequency is set higher than the natural frequency of the system so thatthe operating frequency does not drop below the natural frequency. Insuch a system, the dead time required to achieve ZVS is monitored andthe operating frequency is adjusted such that the dead time is below a

As discussed above, wireless power transmitter 510 determines theminimum operating frequency by monitoring the dead time. Further,wireless power transmitter 510 can determine when the operatingfrequency is below the natural frequency because, in that region, ZVScannot be achieved with any reasonable dead time. Further, wirelesspower transmitter 510 stops lowering or raises its minimum operatingfrequency to maintain operation above, yet close to, the naturalfrequency of the system based upon the dead time information describedabove.

Embodiments of the invention described herein are not intended to belimiting of the invention. One skilled in the art will recognize thatnumerous variations and modifications within the scope of the presentinvention are possible. Consequently, the present invention is set forthin the following claims.

1-9.
 10. A method of operating a wireless power transmitter, comprising:operating a wireless power system that includes the wireless powertransmitter providing power to a wireless power receiver at a firstfrequency; determining a zero-voltage switching (ZVS) deadtime at firstfrequency; lowering the first frequency until the ZVS deadtime is abovea maximum ZVS deadtime; and setting a minimum operating frequency fromthe first frequency.
 11. The method of claim 10, wherein determining aZVS deadtime includes recognizing that the operating frequency placesthe wireless power system in an area where zero-voltage switching is notpossible.
 12. The method of claim 10, further including operating thewireless power transmitter at an operating frequency higher than theminimum operating frequency.
 13. The method of claim 12, furtherincluding determining the ZVS deadtime at the operating frequency and,if the ZVS deadtime exceeds the maximum ZVS deadtime, resetting theminimum operating frequency.
 14. The method of claim 10, wherein themaximum ZVS deadtime is a set parameter in the wireless powertransmitter.
 15. A wireless power transmitter, comprising: an inverterthat includes an upper transistor in series with a lower transistor andcoupled between an input voltage and a ground, the upper transistorbeing driven according to a UG signal and the lower transistor beingdriven according to a LG signal; a TX coil coupled to a switching nodewhere the upper transistor and the lower transistor are connected; acapacitor coupled between the switching node and ground; an analog frontend coupled to receive the input voltage, ground, and a switchingvoltage at the switching node, the analog front end providing signalsthat indicate the level of the switching node; and a processor with aZVS control process, the processor executing instructions to operate thewireless power transmitter to provide power to a wireless power receiverat a first frequency; determine a zero-voltage switching (ZVS) deadtimeat the first frequency; lower the first frequency until the ZVS deadtimeis above a maximum ZVS deadtime; and set a minimum operating frequencyfrom the first frequency that is above the maximum ZVS deadtime.
 16. Thewireless power transmitter of claim 15, wherein to determine a ZVSdeadtime, the processor recognizes that the operating frequency placesthe wireless power system in an area where zero-voltage switching is notpossible.
 17. The wireless power transmitter of claim 15, wherein theprocessor continues to operate the wireless power transmitter at anoperating frequency higher than the minimum operating frequency.
 18. Themethod of claim 17, wherein the processor determines the ZVS deadtime atthe operating frequency and, if the ZVS deadtime exceeds the maximum ZVSdeadtime, resets the minimum operating frequency.
 19. The method ofclaim 15, wherein the maximum ZVS deadtime is a set parameter in thewireless power transmitter.